1. Field of the Invention
The present invention relates to a master medium for magnetic transfer, a magnetic transferring and recording method, a slave medium for magnetic recording, and a method for producing a master medium for magnetic transfer. In particular, the present invention enables accurate transfer of recorded information even when an external magnetic field changes because of some influence during the transfer.
2. Description of Related Art
For writing of a servo signal or the like on a slave medium for magnetic recording (hereinafter simply referred to as a “slave medium”), there has been a known method in which a master medium for magnetic transfer (hereinafter simply referred to as a “master medium”) is fabricated in advance, and then the recorded information on the master medium is transferred to the slave medium. In this method, there is no need to write signals one by one, and thus the write time is significantly reduced. Moreover, in order to reduce the write time during the fabrication of the master medium, there is a proposed method in which concave and convex portions are formed on a non-magnetic substrate, a ferromagnetic material is buried in the concave portion, and a leakage magnetic field corresponding to a predetermined mark is generated by changing the magnetization so as to achieve transfer.
FIGS. 1A and 1B are explanatory views of a conventional master medium. FIG. 1A illustrates an example in which the buried magnetic material is a soft magnetic material. FIG. 1B shows an example in which the buried magnetic material is a perpendicular ferromagnetic material. In FIG. 1A, concave portions 20 are formed in a substrate 1, and a soft magnetic material 4s is buried in the concave portions 20. The substrate 1 is used as the master medium. Meanwhile, a slave medium 5 has been initialized in the direction of magnetization 5m in advance. By applying an external magnetic field in a state where the master medium and the slave medium 5 are juxtaposed, magnetic lines of force 12 produced by the external magnetic field converge on the area of the soft magnetic material 4s and then pass through the slave medium 5. As a result, even in the slave medium 5, an area adjacent to the soft magnetic material 4s is rewritten by the external magnetic field (magnetic lines of force 12). Since the magnetization 5m of the initialized state remains as it is in an area which is not adjacent to the soft magnetic material 4s, the information recorded on the master medium is transferred to the slave medium 5.
In FIG. 1B, the master medium comprises the substrate 1 having a perpendicular ferromagnetic layer 4 buried in the concave portions 20. By applying an external magnetic field in a state where the master medium and the slave medium 5 are juxtaposed, the perpendicular ferromagnetic layer 4 follows the external magnetic field and produces a leakage magnetic field in the direction of increasing the external magnetic field. Therefore, the slave medium 5 receives a strong external magnetic field 14 from the concave portions 20 (the area of the perpendicular ferromagnetic layer 4) and receives a weak external magnetic field 13 from an area other than the concave portions 20 (an area where the perpendicular ferromagnetic layer 4 is not present). For example, when the external magnetic field is adjusted so that the intermediate value between the strength of the strong external magnetic field 14 and the strength of the weak external magnetic field 13 becomes a magnetic field of a strength which inverts the initialized magnetization of the slave medium 5, the initialized magnetization of the slave medium 5 is inverted in the area of the strong external magnetic field 14, but the initial magnetization of the slave medium 5 is not inverted in the area of the weak external magnetic field 13. In this case, the pattern (recorded information) on the master medium corresponding to the concave portions 20 of the substrate 1 is transferred to the slave medium 5.
FIGS. 2A-2D are explanatory views of the magnetic field state and the magnetization boundary shifted amount of a conventional master medium. The same parts as in FIGS. 1A and 1B are designated with the same codes, and the explanation thereof is omitted.
FIG. 2A shows ideal magnetic lines of force 15. FIG. 2B illustrates actual magnetic lines of force 16. In the case of the ideal magnetic lines of force 15, a magnetization boundary 5c of the slave medium 5 is present in a position on a line extended from the perpendicular ferromagnetic layer 4 in a perpendicular direction. In the case of the actual magnetic lines of force 16, since the magnetic lines of force 16 spread, the magnetization boundary 5c of the slave medium 5 changes according to the spread state of the magnetic lines of force 16.
FIG. 2C shows a change of magnetic field in a direction parallel to the substrate surface at the position of the slave medium 5 caused by the spread of the magnetic lines of force 16. The horizontal axis indicates the position from the center of the perpendicular ferromagnetic layer 4 as a relative value. The relative value is expressed by standardizing an end of the perpendicular ferromagnetic layer 4 in a direction parallel to the substrate surface (in theory, the end is the magnetization boundary 5c as shown in FIG. 2A) as 1. The vertical axis indicates a magnetic field at a position of a plane in a direction parallel to the substrate surface, at an arbitrary distance from the substrate 1 toward the slave medium 5. The magnetic field changes in the periphery of the magnetization boundary 5c, and the magnetic field changes in a slanting manner.
FIG. 2D shows the relationship between the change of the external magnetic field and the shift amount of the magnetization boundary. With respect to a slave medium inversion magnetic field Hi, if a magnetization boundary produced by application of an external magnetic field Hn as a reference value is set as a standard, then, when an external magnetic field Hs stronger than the external magnetic field Hn is applied, the magnetization boundary becomes wider outward and shifts outward by an amount ΔSns. On the other hand, when an external magnetic field Hw weaker than the external magnetic field Hn is applied, the magnetization boundary becomes narrower inward and shifts inward by an amount ΔSnw. Thus, when the external magnetic field changes in a slanting manner at a configuration boundary, since the magnetization boundary shifts according to the level of the external magnetic field strength, the configuration boundary does not coincide with the magnetization boundary.
The magnetization boundary becomes a transfer pattern boundary during transfer from the master medium to the slave medium. Accordingly, in order to perform accurate transfer by reducing the positional deviation of the transfer pattern, it is necessary to reduce the change of the transfer pattern boundary. Therefore, it is required to make the state of magnetic field change closer to the theoretical one (FIG. 2A), i.e., to make the change of magnetic field in a direction parallel to the substrate surface steep. In order to realize the steep change of magnetic field, it has been suggested to increase the magnetic field itself, more specifically, to increase the saturation magnetization of the perpendicular ferromagnetic material of the master medium, to increase the layer thickness of the perpendicular ferromagnetic material of the master medium and to bring the master medium and the slave medium more closer to each other. However, each of these solution means has its physical limit.
As described above, in the conventional master medium, the magnitude of the external magnetic field in a direction parallel to the substrate surface is at a slant. Since the transfer pattern boundary (magnetization boundary) changes because of the influence of change of the external magnetic field, an accurate transfer pattern cannot be obtained.
The change of the external magnetic field occurs, for example, when the magnitude of the magnetic field produced by a magnet deviates from a desired value, or when there is a spatial deviation in the interval between the master medium and the slave medium during transfer. The change causes a change in the size of the pattern transferred.